Epoxy resin is excellent in mechanical and chemical characteristics and therefore widely employed in various fields. However, since it is hard and brittle, its usage is limited in spite of its excellent properties. It has been desired to develop an epoxy resin having flexibility.
In order to impart flexibility to the epoxy resin, it is proposed than an epoxy resin be modified with a higher fatty acid, a cyclic ester or a difunctional rubber component. These modified epoxy resins, however, are required to be cured with a curing agent of an acid anhydride at a relatively high temperature, since it is difficult to cure such resins at room temperature. If a polyamine or polyamide is employed as a curing agent for the modified epoxy resin, it can be cured at room temperature. The cured resin, however, does not have enough flexibility and is short of toughness. Further, the epoxy resin prior to curing has a high viscosity and therefore is not good in workability.